This application relates to a process for the refining of shale oil and, more particularly, this application relates to a method for reducing the nitrogen content of shale oil and especially shale oil produced from an in situ oil shale retort.
The term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposits with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. The formation containing kerogen is called "oil shale" herein and the liquid product produced upon decomposition of kerogen is called "shale oil."
Kerogen is considered to have been formed by the deposition of plant and animal remains in marine and non-marine environments. Its formation is unique in nature. Alteration of this deposited material during subsequent geological periods produced a wide variety of organic materials. Source material and conditions of deposition are major factors influencing the type of final product formed.
Kerogen samples, found in various parts of the world, have nearly the same elemental composition. However, kerogen can consist of many different compounds having differing chemical structures. Some compounds found in kerogen have the structures of proteins while some compounds have structures of terpenoids, and others have structures of asphalts and bitumens.
Shale oils produced from oil shale are generally high in molecular weight, viscous organic liquids, which are predominantly hydrocarbonaceous or oxygen, nitrogen and sulfur-containing organic compounds. Shale oils are of varying linear, branched, cyclic and aromatic hydrocarbon and substituted hydrocarbon content, generally having high pour points, moderate sulfur content and relatively high nitrogen content. As a composition of shale oil depends on the composition of the kerogen within the oil shale formation, the composition of the shale oil can vary from one geographic location to another. The shale oil produced from an oil shale formation can vary also between strata within the formation. The nitrogen content of shale oil can also vary dependent on the geographical location of the oil shale deposit from which the shale oil is produced. Such a variance in nitrogen content in different geographical locations can be attributed to differences in the environment during the time of the deposition of the organisms, which upon lithification, become oil shale. Such a variance can also be attributed to the different types of organisms in the separate geographical locations which were deposited to form the organic substance in the oil shale and any organisms within the formed deposit layer which acted upon such deposited material to provide the kerogen within the oil shale formation.
The nitrogen content in shale oil is attributable to basic nitrogen-containing compounds and nonbasic nitrogen-containing compounds. The relative percentages of the basic and nonbasic nitrogen compounds comprising the total nitrogen content of shale oil can also vary depending upon the particular shale oil.
The nitrogen content of shale oil is generally up to about two percent by weight with shale oil from some deposits having greater than two percent by weight nitrogen content. The average nitrogen content of shale oil recovered by in situ retorting of oil shale from the Piceance Creek Basin of Western Colorado is generally between 1.2 and 2.0 percent by weight.
The presence of nitrogen in shale oil presents many problems. For example, nitrogen can interfere with the transportation and use of the shale oil. Deleterious effects, brought about by the presence of nitrogen in shale oil, include: decreased catalyst life in dehydrogenation, reforming, hydrocracking and catalytic cracking reactions; decreased chemical stability of products; and decreased color stability of products. Another problem with the presence of nitrogen in shale oil is that it is undesirable to transport nitrogen-containing shale oil through pipelines which are also used for transporting petroleum products because of possible pollution of such petroleum products with residual nitrogen-containing shale oil in the pipeline. Generally, such petroleum products contain a very low nitrogen content (i.e., greatly less than one percent by weight). The relatively high nitrogen content in shale oil can pollute the pipelines making them undesirable and uneconomical for transporting the low nitrogen-containing petroleum products. In addition, high nitrogen content in shale oil can reduce shale oil throughput in pipelines due to self-polymerization brought about by the reactivity of the nitrogen-containing compounds. Due to the basicity of the nitrogen-containing compounds in shale oil, some corrosion may occur in the pipeline thus causing potential damage to a pipeline which is used to transport shale oil.
Product stability is a problem that is common to many products derived from shale oil, with the major exception of the asphalt cut and those products that have undergone extensive hydrotreating. Such instability, including photosensitivity, is believed to be resultant primarily from the presence of nitrogen-containing compounds.
It is, therefore, desirable to reduce the total nitrogen content of shale oil to increase the utility, transportability, and stability of the shale oil and products derived from such shale oil.
Due to the undesirable nature of the presence of nitrogen in organic fluid streams, such as fluid streams produced in the recovery and refining of petroleum, coal and oil shale, processes have been developed to reduce the nitrogen content to an acceptable level. The level of acceptability for the nitrogen content is generally based upon the end use of the particular fluid stream. The earlier processes for reducing the nitrogen concentration generally relate to removing the basic nitrogen-containing compounds present in the fluid streams. Such processes were developed primarily in regard to the refining of petroleum as the nitrogen content in petroleum is substantially attributable to basic nitrogen-containing compounds. It should also be noted that generally the nitrogen concentration in petroleum is substantially lower than the nitrogen concentration in shale oil. For this reason, in the processing of petroleum, only a small portion of the nitrogen-containing compounds needs to be removed in order to get the total nitrogen content to an acceptable level.
U.S. Pat. No. 3,719,587 to Karchmer et al discloses a process for removing basic nitrogen-containing compounds from coal naphtha. The basic nitrogen compounds are removed by washing the naphtha with water or with a dilute aqueous solution of a strong acid, such as sulfuric acid, hydrochloric acid, phosphoric acid and acetic acid. The dilute acid solutions are from zero to ten percent by weight of the acid.
U.S. Pat. No. 2,848,375 to Gatsis also discloses a method for removing basic nitrogen compounds from organic substances. However, the process used therein is by washing the organic substances with a weak acid in combination with a polyalcohol.
U.S. Pat. No. 2,741,578 to McKinnis teaches that mineral oils can be treated to recover the basic nitrogen-containing compounds by extracting the mineral oil with a selective solvent for the nitrogen bases. The selective solvents taught in the patent are organic hydroxy compounds which have a pH greater than 6.5.
U.S. Pat. No. 2,035,583 to Bailey discloses a process for the separation and recovery of nitrogen bases from mineral oils. In this process, the mineral oils are extracted with a solvent for the basic nitrogen-containing compounds. The acceptable solvents are liquid sulfur dioxide, furfural, aniline, nitrobenzene and isobutyl alcohol. To prevent loss of valuable mineral oils in the solvent, the patent discloses that the resultant extract solution can itself be extracted with dilute aqueous acids to recover the nitrogen bases.
U.S. Pat. No. 2,035,102 to Stratford et al discloses a process for improving the color and viscosity of petroleum oils. The process comprises extracting the oil with phenol, nitrobenzene furfural or liquid sulfur dioxide in combination with an acid, such as picric acid, acetic acid, oxalic acid, citric acid and benzene sulfuric acid.
U.S. Pat. No. 2,541,458 to Berg discloses a process for recovering nitrogen bases from hydrocarbon fractions. In the process the fraction is extracted with a volatile acid or nonvolatile acid salt in combination with a mutual solvent for the acid and the hydrocarbon fraction. The mutal solvents include low boiling alcohols and ketones.
U.S. Pat. No. 2,809,324 to McAlister et al discloses a method for removing nitrogen bases from water insoluble organic solvents, mineral oils and hydrocarbon fractions. In the process a mineral oil is extracted with an aqueous, weak acid solution. The weak acids are classified as acids having dissociation constants below 10.sup.-3.
Many of these earlier processes described in the above patents do not address themselves to the removal of nonbasic nitrogen-containing compounds which can be present in the various organic fluids and which are present in shale oil, in particular, in relatively high concentrations. Additionally, many of the above described processes are not specific for treatment of shale oil and this relatively high nitrogen content found in shale oil. Still further, none of the above processes are specific for lowering the nitrogen content for shale oil produced by in situ retorting of oil shale.